In recent years, Group III nitride semiconductor materials have come to play a more important role in production of LEDs and power devices. Group III nitride semiconductor crystals are known to be produced through metal-organic chemical vapor deposition (MOCVD) or molecular beam expiaxy (MBE), and currently, MOCVD is preferably employed.
However, the MOCVD technique has drawbacks such as high production cost due to use of a large amount of ammonia, and absence of effective organometallic materials containing rare earth metal elements which are to be added to a Group III nitride semiconductor for producing multi-color-light-emitting LEDs.
A Group III nitride semiconductor crystal is known to be also formed through a technique such as molecular beam epitaxy (MBE). The MBE technique has advantages in that a low-impurity-concentration Group III nitride semiconductor can be easily formed, and that a rare earth metal element can be easily added to the semiconductor material.
In the case where a Group III nitride semiconductor crystal is formed through MBE, a Group III element and nitrogen atom vapor are required as raw materials. The Group III element, which assumes the form of solid metal, is generally placed in a crucible made of PBN (pyrolytic boron nitride) and heated in the crucible, to thereby generate atomic vapor. In contrast, nitrogen assumes the form of gas under ambient conditions, vapor of nitrogen atoms is generally generated by, for example, decomposing molecular nitrogen gas or ammonia. In one procedure of forming atomic nitrogen vapor through decomposition of molecular nitrogen gas, a nitrogen radical generator which employs an inductively coupled plasma generated by applying high-frequency power to a coil-form electrode is employed (see, for example, Patent Document 2). In order to enhance the growth rate of a Group III nitride semiconductor by means of a nitrogen radical generator, the nitrogen radical flux density must be enhanced.
Patent Document 1 discloses a radical generator which can generate high-density radicals. The radical generator disclosed in Patent Document 1 has a nitrogen gas supply tube, a CCP unit for generating CCP (capacitively coupled plasma), and an ICP unit for generating ICP (inductively coupled plasma), which are sequentially connected in series. In the plasma generator, a nitrogen gas plasma is formed in the CCP unit before plasma generation in the ICP unit, whereby a high nitrogen radical density can be attained.